1. Field of the Invention:
The present invention relates to a compact vari-focal lens system which has a high vari-focal ratio and is to be used with lens shutter cameras.
2. Description of the prior art:
In the recent years, various attempts have been made to design compact and light-weight zoom lens system along with the market trend of compact cameras. Especially in the field of the lens shutter cameras which do not permit exchange of lens systems, it is strongly demanded to develop a camera equipped with a compact zoom lens system having a vari-focal ratio higher than 2.
Since it is unnecessary prolong a back focal length for the zoom lens system for lens shutter cameras which do not permit exchange of lens systems, it is possible to shorten the total length of the zoom lens system by arranging a lens unit having a strongly negative power at the rearmost location. As the conventional examples of such a zoom lens system, there are known the zoom lens system disclosed by Japanese Patent Kokai Publication No. Sho 62-264019 which consists of two lens units, the zoom lens system disclosed by Japanese Patent Kokai Publication No. Sho 63-153511 which consists of three lens units and the zoom lens system disclosed by Japanese Patent Kokai Publication No. Sho 64-88512 which consists of four lens units. Out of these conventional examples, the zoom lens system which consists of two lens units has a low vari-focal ratio on the order of 2, whereas the zoom lens systems which consists of three and four lens units respectively have a telephoto ratio higher than 1.6 at the wide positions thereof and are not sufficiently satisfactory in the compactness thereof.
In order to design a compact lens system, it is generally sufficient to strengthen powers of lenses by shortening radii of curvature on the lens surfaces. When the powers of the lenses are strengthened, however, the aberrations produced by the lens surfaces are aggravated and the number of the lenses must be increased, thereby making it impossible to design the lens system of interest compact in external design and light in weight.
It is necessary for a zoom lens system to correct chromatic aberration independently in each lens unit thereof. Accordingly, it is general to compose each lens unit so as to comprise at least one positive lens element and at least one negative lens element. Since the lens elements have powers reverse to each other in this case, it is obliged to strengthen the power of each lens element in order to impart the required power to the lens unit. As a result, aberrations are aggravated, thereby making it difficult to design a compact zoom lens system having a high vari-focal ratio.